There are a couple ways to do this.
The easiest way would be to ;
1. Break the ha, partnership
2. on ERC-HA1, simply go to data sources, and highlight the data sources in question, and then select migrate.
Unfortunately, you will NOT be able to migrate historical data unless to take the DR approach as you have already described. If you simply, unplug the heartbeat and main cat5 cable from the erc-ha1, then perform a sync that should do it as well. after syncing, bring your primary back on line. For the life of me I can understand why you'd be doing all this.